Surface resistivity meters are well known, commercially available instruments. Such a meter is equipped with a contact surface which has a geometry adapted for the surface which it is to measure. In one available unit, for example, the instrument includes a contact surface comprising a central disk-shaped electrode and an annular electrode positioned around the disk and spaced apart from it. A voltage is impressed between the two electrodes. Electric field lines originate at the annulus and terminate at the disk producing currents which are indicative of the surface resistivity of the material with which the instrument is in contact.
One problem with an arrangement of this type is that frequently the disk and annulus do not make proper contact with the surface to be measured. The surface, for example, may not be planar. Consequently, complete contact is impossible and incorrect resistivity readings are obtained. Problems of this kind are overcome to some extent by providing a flexible surface material on the disk and annulus so that they can conform to the surface to be measured. Such materials are ionic conductors which make the instrument relatively insensitive. Consequently, such instruments require relatively high voltages, about 1000 volts, for operation because they need to have relatively high currents to measure.
Moreover, available resistivity meters provide readings to several digits times 10 to a power. Circuitry to provide such a reading is expensive and the reading is misleading because it offers a degree of confidence which is not supported by the accuracy of the measurement.
Surface resistivity meters typically comprise an enclosure and the contact surface is connected to a cable. The enclosure is of considerable size and the cable is often inconvenient.